VPN Question

I have a site to site vpn currently working and a policy to allow a device on my dmz port to connect to a remote device.The policy has source nat enabled. I want to add another policy to allow a device on the trust port to connect to the same remote device using this existing tunnel. It wont allow the policy because the ike id already in use on the first policy when I enable source nat. How do I add the policy so that the device on the trust port can utilize the vpn also?